The lone-term goal of the work described in this proposal is to understand how growth factors and hormones effect the expression and function of ion channels. The proposal will test the specific hypothesis that growth hormone (GH) and/or insulin like growth factor 1 (IGF-1) are important regulators of the T-type calcium current in atrial myocytes. Both electrophysiological and molecular biological techniques will be used to study the expression of T-current in myocytes. AIM 1: The direct effects of GH and IGF-1 on T-current expression will be studied in cultured myocytes. Use of an in vivo system allows precise control of the composition of the cell environment thus simplifying to some extent experimental design. The role of second messenger pathways and of intracellular processes such as protein synthesis, RNA translation, and DNA transcription will be determined. AIM 2: Postnatal changes in the density and biophysical properties of T-type calcium current will be determined in myocytes acutely isolated from a growth hormone deficient (GHD) rat. A naturally occurring "knockout", this mutant provides a unique whole animal model to assess the effects of GH and IGF-1 on T- current expression. Reintroduction of GH and IGF-1 by osmotic minipumps will allow separation of the effects of these agents in the complex environment represented by the intact animal. AIM 3: Molecular biological approaches will be used to clone the T-channel alpha-subunit. Probes will be constructed and the relationship between current density and mRNA levels investigated to provide additional information about the regulation of T-current expression. AIM 4: The correlation between the expression of different beta-isoform subunits and T-current density will be determined to investigate the possible involvement of this subunit in regulating T-current density. The results will provide new information about the long-term regulation of electrical excitability in cardiac tissue both during normal postnatal development and in response to altered physiological states. While the proposal is focused on T-current in cardiac myocytes, this calcium current is widely distributed in neuronal and neural secretory tissues. The information gained should therefore have broad applicability to other excitable cells.